The invention relates to a method for synchronizing RF antenna signals of a plurality of RF antenna sites arranged at different locations of a radio transmission system. The invention also relates to a radio transmission system adapted for performing the method.
Coherent network MIMO (Multiple-Input/Multiple-Output) offers a significant increase of spectral efficiency in radio transmission systems such as cellular networks, especially in systems with frequency re-use in which the same spectrum is used in each cell. In this case, the system performance is normally limited by inter-cell interference.
In order to draw maximum benefit from coherent network MIMO transmission in the downlink direction, i.e. from the RF antenna sites to the mobile stations, antennas located at distant antenna sites (e.g. located in a plurality of co-operating base stations or Remote Radio Heads of the same base station) should transmit radio signals with correlated phases (acting as “calibrated Antennas”).
For this reason, a method to maintain the synchronization of the RF antenna signals is desired that limits deviations between the RF antenna signals (phase jitter) to less than a fraction of the RF period over a time frame in the order of some 100 ms. This period would be long enough to allow feedback mechanisms to control the phases. The carrier frequency of the radio signals is typically between 1 and 5 GHz for cellular applications and the spacing between the antennas can be in the order of e.g. 500 m to 1 km or even more for a macro-cellular environment.
The known methods for synchronisation of base stations are e.g. based on using the Ethernet backhaul link or alternatively on using a GPS clock reference, both of which will be shortly described in the following:
Use of IEEE 1588 or CPRI interface (Ethernet backhaul link):
In the category of Ethernet-based (or protocol based) synchronization there is one method according to IEEE 1588 and another one based on the CPR) interface. Methods of this category can reach synchronization down to a fraction of a microsecond, but they do not allow to maintain distant antennas calibrated to the above requirements.
Use of a GPS reference:
In the GPS case, the master clock (master oscillator) is located in the satellite of the GPS system and a 10 MHz reference signal is provided by the OPS satellite receiver unit. A GPS receiver is installed at each antenna site providing the signal that controls the oscillators.
However, both approaches, i.e. GPS and IEEE 1588, are not precise enough also for an another reason, resulting from the fact that Phase Locked Loops (PLLs) are used to generate the RF carrier signal from reference signals with much lower frequency:
Assuming a 2 GHz RF signal will be derived using a PLL from a 10 MHz reference signal, a PLL inband phase noise of 20 log(20 Hz/10 MHz)=46 dB will be achieved. However, the present RF-radio channel grid is in the region of 100 KHz up to 1 MHz and hence the PLL in-band phase noise will increase e.g. at a 100 KHz reference frequency to 86 dB. This will lead to a huge uncorrelated phase deviation within the individual RF-(LO) Oscillators in any Remote Radio Head, joined with an unwanted and non-correlated radio pattern (similar to a SDMA (Space Division Multiple Access) pattern) on the air interface.
As a result, all antennas that are co-operating in the transmission process require precise synchronisation using a signal derived from a master oscillator with a frequency in the order of the carrier frequency.
There is one commercially available solution for performing such a synchronization by using a GPS signal combined with a Rubidium (Rb) clock: In this case, the very precise (rubidium) clocks are synchronized externally by GPS signals. However, due to use of Rubidium clocks, this solution is expensive, which prohibits its use for many applications.